A heart contracts and relaxes, that is to say, beats at a regular rhythm. Arrythmia is a serious disease that a period of this cardiac beat becomes irregular, sometimes causing cardiac arrest. Various and detailed studies have been carried out on the cardiac beat mechanism for medical treatment and diagnosis of arrythmia.
Cardiac contraction occurs as follows. First, electrical impulses are emitted at a constant period from a part of a right atrium, which part is called a sinoatrial node. The electrical impulses are passed to cardiac muscle cells of the right atrium and cardiac muscle cells of a left atrium. Consequently, myofibrils in the cardiac muscle cells contract. When this contraction of the myofibrils occurs all over the right atrium and the left atrium, the right atrium and the left atrium are caused to contract. Further, a part of the electrical impulses reaches an atrioventicular node located below the right atrium and in the vicinity of an interventricular septum. After reaching the atrioventicular node, the impulses pass through His bundles, right and left bundle branches, and Purkinje fibers, and then reach a left ventricle and a right ventricle, causing the left ventricle and the right ventricle to contract. As the foregoing discusses, a cardiac beat is caused by electrical impulses passing through the heart.
The cardiac muscle cells are in the shape of a cylinder with a diameter of approximately 5 to 20 μm and a length of approximately 100 μm. The cardiac muscle cells are arranged in a certain orientation to form a bundle. An orientation of the length of the cardiac muscle cells is same as that of the myofibrils in the cells, and therefore is called a fiber orientation. Muscle contraction is caused by sliding movement of the myofibrils. The fiber orientation is closely related to cardiac contraction movement. Therefore, the fiber orientation is an important factor in mechanically analyzing the cardiac contraction. Further, an electric current passes easily in the fiber orientation in the cells. The fiber orientation relates to a conduction orientation of the electrical impulses in the heart. Therefore, the fiber orientation is an important factor also in analyzing conduction pathways of the electrical impulses in the heart.
It is empirically known that appropriate fiber placement is important for efficient cardiac contraction and blood pulsation. The fiber orientation varies in different parts. The fiber orientations of the entire heart are complex. Conventionally, the fiber orientation is measured by anatomical and histological methods. In view of ethics, a heart of a dog or a pig, which are relatively close to a human, is utilized in place of a human heart.
For example in Documents 1, 2, the fiber orientation and the sheet orientation of a pig heart are measured, and this measured fiber orientation data is organized with introduction of three coordinate systems, such as an ellipse coordinate system, and Hermitian finite element. The sheet orientation is in connection with a plane (sheet) where the cardiac muscle cells are arranged. Mathematically, the sheet orientation is vertical to the plane.
Documents 3 and 4 disclose a method of measuring and calculating a fiber orientation with the use of diffusion tensor magnetic resonance imaging (MRI). A spatial distribution of the fiber orientation of a dog heart is actually obtained, and is compared with histological data for verification.
The foregoing results of measurement have roughly clarified a pattern of the fiber placement in the heart. Findings from animals are utilized to creates a virtual human heart model in a calculator, and attempts to contribute to medical care and drug discovery have been made by simulations and the like.
However, no modeling device has been realized by which information on the fiber orientation of the cardiac muscle cells, which information is obtained from an animal, is buried in a human heart to perform modeling suitably. This is due to the following reasons.
First, no coordinate system suitable to specify the spots in a heart has not been found. For example, to apply a fiber orientation obtained from animal onto a human heart model, it is necessary to establish a one-by-one correspondence between a spot in the animal heart and a spot in the human heart. However, the shapes of the hearts vary among species, and further, among individuals. Furthermore, the shapes of the hearts are very complex. Therefore, it is extremely difficult with an ordinary XYZ-axes orthogonal coordinate system or the like to set a correspondence between spots of two different hearts. In view of the foregoing circumstances, there have been demands for a modeling device by which characteristic information, such as a fiber orientation, obtained from an object is easily projected onto a differently-shaped object, even if the shape of the object, from which the characteristic information is obtained, is complex, such as the shape of a heart.
Further, no method of suitably setting a local coordinate system to define the fiber orientation and the like at a spot in a heart has been found. The fiber orientation, for instance, is closely related to the outer shape of the heart. For example, the fiber orientation at a point on a surface of the epicardium of the heart is included within a plane that is in contact with the point. However, if the fiber orientation information is expressed with the use of an ordinary global coordinate system, it is not possible to express the fiber orientation in such a way as to correspond to the outer shape of the heart, because the global coordinate system has no relationship with the outer shape of the heart. Therefore, if the fiber orientation data obtained from animals is directly applied to the human heart, contradiction may arise in the fiber orientation. For example, the fiber orientation protrudes from the epicardium of the heart. Further, even if a hypothesis about the fiber orientation on the basis of the findings obtained from the animal heart is to be applied, it is not possible to apply the hypothesis naturally. In view of the foregoing reasons, the local coordinate system needs to be set at respective spots in the heart. The heart, however, has a very complex shape. Setting the local coordinate system by performing a geometric calculation each time on the basis of the shape of the heart requires a vast amount of calculation and is therefore not realistic. In view of the foregoing circumstances, there have been demands for a modeling device by which orientation characteristic information, such as a fiber orientation, that is related to an outer shape of a heart is easily projected from an object onto another object.
(Document 1)
Stevens C, Hunter P J. Sarcomere length changes in a 3D mathematical model of the pig ventricles. Prog Biophys Mol Biol. 2003 May-July; 82(1-3): 229-241.
(Document 2)
Stevens C, Remme E, LeGrice I, Hunter P. Ventricular mechanics in diastole: material parameter sensitivity. J Biomech. 2003 May; 36(5): 737-748.
(Document 3)
Scollan D F, Holmes A, Winslow R, Forder J. Histological validation of myocardial microstructure obtained from diffusion tensor magnetic resonance imaging. Am J Physiol. 1998 December; 275(6 Pt 2): H2308-H2318.
(Document 4) Scollan D F, Holmes A, Zhang J, Winslow R L. Reconstruction of cardiac ventricular geometry and fiber orientation using magnetic resonance imaging. Ann Biomed Eng. 2000 August; 28(8): 934-944.